Generally, when an electron tube such as cathode ray tube (CRT) or the like is operated under a condition where a vacuum pumping from inside of the electron tube is insufficient, such insufficient vacuum degree has an adverse influence on characteristics of the electron tube. Therefore, in the electron tube, there is provided a getter device for sufficiently removing unnecessary gas components from the inside of the electron tube.
In recent years, for example, in a technical field of television (TV) for civil applications, a large-screen TV set having a screen size of about 32 to 37 inches has become widely used. Under this situation, the CRT to be used for such the large-screen TV has been increased in size thereof. In such a large-sized electron tube, number of, parts to be assembled into the tube is also increased. Simultaneously, a volume of the electron tube is also further increased.
Therefore, it becomes more and more important to improve the characteristics per se and stability of the characteristics of the getter device for maintaining a high degree of vacuum in the electron tube by adsorbing both: a residual gas remained in the electron tube after completion of the vacuum-pumping operation using a vacuum pump in the electron tube manufacturing process; and the unnecessary gas components released from respective parts including a vacuum chamber arranged in the electron tube.
Concretely, for the purpose of absorbing the unnecessary gasses released in the electron tube, the electron tube typically represented by CRT is equipped with an evaporation type getter device which is manufactured by: preparing a getter material consisting of a mixture of Ba—Al alloy powder containing Ba, Al as main component and Ni powder; and filling the getter material into a metal container formed of an alloy such as iron, steel, Ni alloy, stainless steel.
In this getter device, when the getter material consisting of the mixture of Ba—Al alloy powder and Ni powder is heated to rise a temperature thereof, an exothermic reaction of bringing Al component and Ni component into combination is started at a predetermined constant temperature, Ba component contained in the getter material is vaporized (getter-flashed), and an impurity-gas adsorbing function is exhibited by the Ba component. As a result, a degree of vacuum in the electron tube can be maintained to a predetermined value.
Accordingly, the getter device equipped in the electron tube is required to evaporate a predetermined amount of Ba (barium) by which the unnecessary gasses contained in the electron tube is absorbed thereby to increase the degree of vacuum in the electron tube. Therefore, when the amount of the evaporated Ba is small, the predetermined degree of vacuum required for the electron tube cannot be obtained.
On the other hand, when the amount of evaporated Ba is excessively large, Ba-amount to be adhered to structural members such as inner wall or the like of the electron tube also becomes excessively large. Therefore, the excess amount of Ba becomes a cause of inviting troubles such that an abnormal discharge is liable to occur and a part of Ba adhered to the inner wall is dropped and adhered to another portion within the electron tube thereby to obstruct a normal operation of the electron tube. Accordingly, it is technically important to control the amount of evaporated Ba so as to be within a predetermined range required for the electron tube.
However, in the conventional getter materials, although a temperature at which the exothermic reaction of the getter material starts has a great influence on Ba evaporation amount, such a technical fact has not been recognized at all. In addition, needless to say, a range of the exothermic reaction starting temperature has not been clearly determined to a specified range. Therefore, a dispersion or scattering of the exothermic reaction becomes large, so that there has been posed a problem that the Ba evaporation amount cannot be sufficiently controlled.
Further, in general, the getter material is filled into a metal container having an opening portion called a getter ring for evaporating the Ba component, and the getter material is actually used in the packed state. However, when the exothermic reaction starting temperature is excessively high, there has been also posed problem that a thermal deformation and melting of the metal container per se are liable to occur, so that the evaporation of Ba cannot be performed with a stable condition.
As a method of evaporating Ba from the getter device equipped in the electron tube, there has been generally adopted a method in which a predetermined radio frequency induction generated by radio frequency induction power is applied from an outside of the electron tube to the getter material disposed in the electron tube under a non-contacting state thereby to heat the getter material.
According to this heating method, there can be provided tangible advantages that the getter material can be heated under a condition where the heating operation would not affect to other portions except the getter device in the electron tube sealed in a vacuum condition, and it is easy to rapidly heat the getter material whereby a process time required for producing the electron tube can be shortened, thus being advantageous for the process of manufacturing the electron tube.
However, in the above heating method by applying the high-frequency magnetic field, the metal container filled with the getter material is also heated at the time of heating the getter material. At this time, a relationship between a specification of the metal container and radio frequency induction heating conditions have not been paid attention at all in the conventional getter device, so that there have been also posed the following problems. Namely, in a case where a ratio for heating the metal container by the radio frequency induction power is remarkably larger than that for heating the getter material, a temperature rise of the metal container becomes greatly larger than that of the getter material, so that the metal container is easily deformed and molten before the exothermic reaction of the getter material is started.
As a result, there is posed a problem such that it becomes difficult to stably evaporate the getter component, and the temperature rise of the getter material per se is delayed. In addition, it requires a long heating-evaporation process time for the getter material to obtain a sufficient evaporation amount of Ba so as to attain a predetermined vacuum degree in the electron tube. In other words, the evaporation amount of Ba corresponding to the heating time and responsiveness until the predetermined degree of vacuum is obtained are lowered, thus being a bottleneck problem. On the other hand, there has been also posed a problem that when the heating-evaporation process time is set to be short, the amount of evaporated Ba is insufficient, so that it becomes difficult to obtain the vacuum degree required for the electron tube.
The present invention had been achieved to solve the aforementioned problems, and an object of the present invention is to provide a getter material, an evaporation type getter device and an electron tube capable of suitably controlling an evaporation amount of getter components under a stable condition, and is excellent in responsiveness because a time ranging from a starting time of heating the getter material to a starting time of the evaporation of the getter components is short. (i.e., the evaporation amount of Ba corresponding to the heating time and responsiveness until the predetermined degree of vacuum is obtained are excellent.)
Another object of the invention is to provide an evaporation type getter device and an electron tube using the getter device in which the metal container to be filled with the getter material is free from deformation and melting, and a heat-evaporation process time of the getter material can be shortened, so that there can be provided the evaporation type getter device excellent in responsiveness because a time required for the electron tube to attain to a predetermined vacuum degree can be also shortened.